1. Technical Field
This invention relates generally to ignition devices, and more particularly to spark plugs having an integral combustion sensor.
2. Related Art
It is known to incorporate a sensor in a spark plug assembly to sense the combustion pressure within a cylinder bore during use of the engine. By being able to sense the magnitude of the combustion pressure, it can be determined if the engine is operating as intended, or if service is needed. Unfortunately, current integral sensor spark plug technology is limited in its ability to provide an accurate measurement of the combustion pressure due to various unwanted influences exerting forces on the sensors in addition to the combustion gas forces. These unwanted forces applied to the integral sensor generally occur during installation and/or while the engine is in operation. Accordingly, the pressure measurements obtained by the sensors may not be a true and accurate indication of the combustion forces.
As shown in FIG. 1, a known spark plug assembly 1 has an integral sensor 2 for sensing combustion forces within a cylinder bore. The assembly 1 has a ceramic insulator 3 fixed within a metal outer shell 4. The insulator 3 is fixed under compression between a turnover 5 of the shell 4 and an internal gasket 6. The compression exerted on the insulator 3 acts to provide a reliable seal at the gasket 6 during operating conditions. The sensor 2 is maintained under compression between an external flange 7 of the shell 4 and a hexagonal fitting 8. In operation, combustion forces exerted on a core nose 9 of the insulator 3 pushes the insulator 3 upwards axially away from the cylinder bore, wherein some of the force is transferred through the shell turnover 5 into the shell 4, which in turn, acts to stretch a flexible upper section 10 of the shell 4. As the flexible upper section 10 is stretched, the compression force imparted on the sensor 2 is reduced, which is interpreted as a direct correlation of the combustion force within the cylinder bore. However, during installation of the spark plug assembly 1 into the cylinder block, a tensile force is established between an external thread region 11 of the shell 4 and a gasket sealing face 12 on the bottom sealing surface of the external flange 7. The magnitude of the tensile force can vary from one assembly to another as a result of tightening torque tolerances and during normal use due to, for example, temperature fluctuations or uncontrolled deformations of the cylinder head. Regardless of the magnitude of the tensile force, the tensile force tends to reduce the force that compresses the insulator 3 between the turnover 5 and the gasket 6 in an uncontrollable and unknown manner. As such, the tension exerted during combustion on the flexible section 10 is reduced, which directly affects the “as assembled” preload force applied on the sensor 2 in an unknown and uncontrollable fashion. Thus, the force magnitude indicated by the sensor 2 during combustion may not be an accurate indication of the compression pressure within the cylinder bore.